This invention relates to electric motors, and, more particularly, to electrically braked alternating current motors.
The prior art is replete with disclosures for electrically braked alternating current motors. For example, Noodleman, U.S. Pat. No. 2,627,059, discloses an electrically braked alternating current motor having a first single phase brake winding wound in the slots of the alternating current motor stator with the run winding and so arranged that the brake winding forms a greater number of poles than the run winding. The larger the number of poles formed by the brake winding, the greater will be its effectiveness in braking the motor.
Also in accordance with U.S. Pat. No. 2,627,059, two single phase brake windings are provided, which are phase displaced by 90.degree., with one of the brake windings having a capacitor or short circuit thereacross. While an increase in braking effect is produced by a second, capacitor-shunted or short-circuited brake winding, should the first brake winding fail, such as due to burn out from overheating, the alternating current motor can no longer be electrically braked.
Voege, U.S. Pat. No. 2,808,552, discloses an electrically braked reversible alternating motor current motor which includes a main brake winding and an auxiliary brake winding. The circuit control means includes a reversing switch to energize the auxiliary brake winding in accordance with the direction of rotation of the reversible alternating current motor, the main brake winding and the auxiliary brake winding being simultaneously energizable to produce the braking effect. If either the main brake winding or the auxiliary brake winding fails, however, such as due to burn out from overheating, the electrical braking effect can no longer be produced.
Experience has shown that the normal life expectancy of a brake winding, such as included in the above-cited prior art electrically braked motors, is approximately one year. Stated differently, the brake winding incorporated in prior art electrically braked alternating current motors can be expected to burn out on the average of approximately once a year. When the brake winding fails, the alternating current motor can no longer be electrically braked, and must be replaced. The resulting downtime can cause significant economic loss. For example, if the motor were employed as a drive on an assembly line or machine tool, production would have to be shut down until such time as the brake winding was replaced or a different motor substituted. Even if downtime could be tolerated until time was found to repair or substitute, yearly replacement of the brake winding results in a high maintenance cost.